CATALYTIC ASYMMETRIC INTRAMOLECULAR PAUSON-KHAND AND PAUSON- KHAND-TYPE REACTIONS

1981. 4 The original protocol for the PKR required superstoichiometric amounts of a metal complex, CATALYTIC ASYMMETRIC INTRAMOLECULAR PAUSON-KHAND AND PAUSON- KHAND-TYPE REACTIONS Reported by Steven G. Ballmer October 9, 2008 INTRODUCTION First reported in 1973 by I. U. Khand and P. L. Pauson, the cobalt-mediated carbonylative [2+2+1] cycloaddition of an alkyne, 1, an alkene, and carbon monoxide to afford a cyclopentenone, 2, is referred to as the Pauson-Khand reaction (PKR) (Scheme 1). 1,2 Since its inception, the PKR has gained interest in the synthetic community and shows increasing use in syntheses of cyclopentenones. 3 Scheme 1. Inter- and Intramolecular PKRs. One of the limitations of the PKR is the poor regioselectivity observed in intermolecular reactions where unsymmetrical alkynes and alkenes can give rise to mixtures of four constitutional isomers. Inherent regiocontrol of the intramolecular PKR of enynes 3 to form bicyclic cyclopentenones 4 (Scheme 1) makes this variant appealing and it has been actively investigated since its inception in which limits its utility. The first catalytic PKR was reported in 1994 5 and shortly thereafter the first catalytic PKR where a metal other than cobalt, titanium was used. 6 Further development has produced catalytic, intramolecular PKRs that employ ruthenium 7 and rhodium(i) 8 complexes as catalysts. Although the catalytic, asymmetric, and intramolecular PKRs were developed separately, the demand for a combined, catalytic, asymmetric variant was clear. This demand was met with the report of the first catalytic, asymmetric intramolecular PKR. 9 Here, a chiral titanocene complex is employed in the enantioselective intramolecular construction of a variety of bicyclo[3.3.0]octenones 4 (Z = CR 2, O, NR) from 1,6-enynes 3 (Z = CR 2, O, NR). Copyright 2008 by Steven G. Ballmer

Discussed herein are the critical advancs and recent developments toward catalytic asymmetric intramolecular PKRs including the development of new transition-metal, catalytic complexes, adaptation of these systems to a broader substrate base, and applications of the PKR in complex molecule synthesis. MECHANISM Although a detailed mechanism for the PKR has yet to be fully elucidated, the pathway originally suggested by Magnus for the dicobalt octacarbonyl-mediated intramolecular PKR is widely accepted. 10 In this mechanism it is suggested that the alkyne and cobalt catalyst form a stable alkyne- [Co 2 (CO) 6 ] 18-electron complex. Subsequently, the complex suffers loss of a single CO ligand from one Co atom, affording a 16-electron alkyne-[co 2 (CO) 5 ] complex. This complex then coordinates the olefin, reforming an 18-electron complex that undergoes alkene insertion to afford the cobaltacycle. Subsequent insertion of CO and reductive elimination affords the cyclopentenone and [Co 2 (CO) 6 ]. Pioneering work in mechanistic elucidation shows that isolation of the decarbonylated alkyne-cobalt complex before alkene association is possible if sulfur-containing ligands are used for stablization 11a and that the argon-matrix-trapped decarbonylated complex is detectable by IR spectroscopy. 11b This basic mechanistic formulation has been adapted to explain the CO pressure-dependent selectivity observed in asymmetric, intramolecular PKRs (Scheme 2). In this cycle, chiral transition metal catalyst 5a, which can be in equilibrium with 5b under a carbon monoxide atmosphere, undergoes coordination by enyne 3 to afford enyne/catalyst π-complex 6a which can be in equilibrium with 6b. Subsequent oxidative addition/cyclization affords metallacyclopentene 7 stereoselectively. This complex undergoes CO insertion to give metallacycohexenone 8 followed by reductive elimination to afford bicyclic cyclopentenone 4 thereby regenerating 5a. The suspected equilibria of 5 and 6 suggest that at higher CO pressures both 5b and 6b are favored. As 5b is less reactive than 5a and oxidative cyclization of 6b would give diminished stereoinduction this proposed cycle rationalizes the coupling acceleration and increased stereoselectivity observed at lower CO pressures.

Scheme 2. Proposed Mechanism for Intramolecular PKR. More recent studies have shed light on the mechanism of the PKR and support the current hypotheses. Specifically, tandem mass spectrometry studies have detected the decarbonylated cobaltalkyne complex before association or insertion of the alkene, providing for the first time, solid evidence of the proposed mechanism. 12a Furthermore, x-ray crystal analysis of isolated intermediates have provided the first characterized η 2 -alkene-pentacarbonyldicobalt-alkyne complex, again supported the propsed mechanism. 12b DEVELOPMENT OF CATALYTIC ASYMMETRIC PAUSON-KHAND REACTIONS The first catalytic, asymmetric, intramolecular PKR employed an enantiomerically pure dicarbonyl titanocene complex with an ethylene bistetrahydroindenyl ligand ((S,S)-(EBTHI)Ti(CO) 2 ), produced in situ from (S,S)-(EBTHI)Ti Me 2. 9 With this complex a variety of 1,6-enynes afford bicyclic cyclopentenones in high isolated yields (70-94%) and good enantioselectivities (49:1 to 6:1 er) with as little as 5 mol % catalyst loading. This complex allows for reasonable functional group compatibility as 1,1-disubstituted olefins, ethers, esters, and amines are tolerated. Stereoinduction is believed to arise from the formation of titanocycle 9 (Chart 1). The enyne in 9 adopts a favorable conformation during olefin complexation, reducing the steric strain with the EBTHI ligand that the diastereomeric titanocycle possesses. The first example of a cobalt-catalyzed, asymmetric PKR employed a dicobalt phosphine complex formed in situ from Co 2 (CO) 8 and (S)-BINAP. 13 Here, it is rationalized that stereoinduction arises from the formation of dicoblatacycle 10 (Chart 1) where steric strain requires the bidentate phosphine ligand to coordinate both cobalt catalyst atoms separately. The boat-like conformation shown in 10 is favored as the chair-like conformer causes steric strain between the phosphorous phenyl rings

and the opposing naphthyl rings. Furthermore, the alkyne R substituent is favorably positioned to reduced steric interaction with the phenyl rings and the substrate backbone is positioned similarly, giving rise to olefin face selection. Chart 1. Transition Metal Complexes in PKR. More stable catalysts with defined structures were sought to address the expansion of late transition-metals and tunable phosphine ligands in PKR catalytic systems. Cationic rhodium (I) phosphine complex [Rh(CO)(S)-BINAP] + generated in situ from [RhCl(CO) 2 ] 2 and an excess of (S)- BINAP in the presence of silver triflate was first employed in 2000. 14 The inclusion of a coordinating solvent (THF) is necessary for high enantioselectivity and at 1 atm of carbon monoxide a variety of 1,6- enynes afford bicyclic cyclopentenones in yields as high as 99% and enantioselectivity as high as 49:1 er. Here, stereoinduction arises from the formation of rhodacycle 11 (Chart 1). This diastereomer of 11 is preferred as the R substituent on the alkyne occupies the less sterically encumbered open quadrant created by the (S)-BINAP ligand. The first iridium catalyst employed for the asymmetric PKR, [IrCl(CO)(S)-tolBINAP] is also prepared in situ from [Ir(COD)Cl] 2 and (S)-tolBINAP. 15 With this catalyst, stereoinduction is rationalized also by the formation of iridacycle precursor 12 (Chart 1). The alkyne substituent and alkene are positioned favorably in the open quadrants created by the ligand, providing the observed stereoselectivity. EXPANSION OF SUBSTRATES FOR PAUSON-KHAND REACTIONS Allenynes The demand for a greater substrate scope stimulated the development of an allenynyl PKR where an allene moiety replaces the olefin of the common enyne. 16 Allenyne 13 inherently creates two pathways for an impending PKR (Scheme 3). In one path, catalyst complexation occurs at the internal

allenic π-bond (14a), affording bicyclic methylenecyclopentenone 15a after cyclization. The second path affords bicyclic dienone 15b after catalyst complexation to the external allenic π-bond (14b). Although catalytic asymmetric methods for allenynyl PKRs remain largely unexplored and regioselectivity can cause issues, the inclusion of allenynes to PKR substrate base opens a broad avenue toward diversified PKR products. Scheme 3. Two Pathways for Allenynyl PKR. Dienes and Ene-allenes Wender and co-workers reported the first use of a dienyne in an intramolecular PKR. 17 Subsequent reports described the use of ene-dienes 18 as well as diene-allenes 19 in then-unprecedented catalytic, asymmetric, intramolecular PKRs. A series of 1,3-dien-8-ynes similar to 16 (Chart 2) undergo [2+2+1] cycloaddition in the presence of CO and a rhodium catalyst to afford isoprenylated bicyclo[3.3.0]octenones as single diastereomers in isolated yields as high as 96%. Likewise, a variety of 1,3,8-trienes similar to 17 (Chart 2) undergo [2+2+1] cycloaddition in the presence of CO and a rhodium catalyst to afford bicyclo[3.3.0]octanones as single diastereomers in good yield. A series of dieneallenes such as 18 (Chart 2) afford bicyclic isoprenylated methylenecyclopentanones diastereoselectively in good yield under standard PKR conditions. Finally, ene-allenes have also been conscripted into use in the PKR. 20 Here a variety of ene-allenes similar to 19 (Chart 2) combine with CO under rhodium catalysis to afford the corresponding phenylsulfonylbicyclo[4.3.0]nonenones in a diastereoselective fashion and good yield after isomerization of the initial product to the α,β-unsaturated ketone. Chart 2. Dienyl and Allenic Structures.

APPLICATIONS IN COMPLEX MOLECULE SYNTHESIS Total Synthesis of Isocarbacyclin In an illustration of the power of the PKR, Saito and coworkers reported the diastereoselective, total synthesis of isocarbacyclin (22) (Scheme 4), a metabolically stable analogue of the vasodilator prostacyclin. 21 Bicyclo[3.3.0]octenone intermediate 21 (an obvious PKR product) was targeted as the key precursor for 22. Dioxolane 20, available in a few synthetic steps from L-ascorbic acid, is treated with 5 mol % dicobalt hexacarbonyl-(2-methyl-3-butyn-2-ol) complex in the presence of triethylsilane (Et 3 SiH) and cyclohexylamine (CyNH 2 ) in DME under 1 atm of carbon monoxide to afford 21 as a single diastereomer in 76% yield based on a 67% conversion (Scheme 4). Although under these conditions complete conversion of 20 was not achieved, the isolation of 21 as a single isomer suggests that the reaction provides highly efficient discrimination over not only the diastereotopic allyl groups, but also the faces of the double bonds in 20. Here the authors suggest that the protected diol acts as a prominent stereocontrolling unit in the expected chair-like transition state. Scheme 4. Synthesis of Isocarbacyclin. Synthesis of (-)-Pentalenene Fox and Pallerla reported the first enantioselective synthesis of (-)-pentalenene (25) (Scheme 5), the unnatural enantiomer of the angular triquinane natural product pentalenene. 22 The ability of the intramolecular PKR to install stereodefined quaternary centers led to the identification of cyclopropanefused bicyclo[3.3.0]octenone intermediate 24 (Scheme 5) as a key intermediate. Upon treatment of enantiomerically-enriched cyclopropene 23 with dicobalt octacarbonyl (60 mol %) in the presence of tetramethylthiourea (TMTU) in toluene under 1 atm of carbon monoxide, 24 is obtained in 64% yield after separation from its diastereomer (4:1 dr) (Scheme 5). As expected, the key quaternary center is installed stereoselectively. This result, however, comes only after a thorough investigation of other PKR

promoters. The use of N-oxides proves unproductive as only unreacted 23 is isolated and while n-butyl methyl sulfide affords 24 as a single diastereomer it does so in a relatively low 45% yield. Scheme 5. Synthesis of (-)-Pentalenene. Total Syntheses of Uncommon Sesquiterpenoids Drawing on the development of the allenenyl PKRs, Mukai and coworkers reported the first total synthesis of cyclopropane-fused cis-perhydroindane 28 (Scheme 6), an uncommon tricyclic sesquiterpene isolated from Jatropha neopauciflora. 23 Enroute to 28, bicyclo[4.3.0]nonenone 27 (Scheme 6) represents a promising PKR adduct. Retrosynthetically, allenene 26 (Scheme 6) serves as the required target for the planned intramolecular PKR. Beginning with the readily available dimethyl D-tartrate, 26 is obtained in good fashion and subsequently subjected to PKR conditions. Treatment of 26 with cationic [RhCO(dppp) 2 ]Cl (5 mol %) in toluene at reflux under a carbon monoxide atmosphere affords 27 in 74% yield as a single isomer. Here, stereoinduction can be rationalized by the formation of the expected rhodacycle. Scheme 6. Synthesis of Sesquiterpenoids. CONCLUSION AND OUTLOOK The formal [2+2+1] cycloaddition of an alkyne, an alkene, and carbon monoxide, know as the Pauson-Khand reaction, represents one of the most versatile synthetic routes to cyclopentenones. In addition, the intramolecular variant provides efficient access to fused bicyclic cyclopentenone structures in an atom economical fashion. The recent development of late transition metal catalysis with chiral

complexes has pushed the Pauson-Khand reaction to the mainstream of organic synthetic methods. Newer adaptations of dienyl and allenyl moieties have further expanded the scope of this cycloaddition and its frequent appearance in current literature suggest that it may soon be the method of choice for cyclopentane syntheses. The utility of the intramolecular Pauson-Khand reaction in complex molecule synthesis is still largely confined to non-catalytic and non-asymmetric methods and relies largely on substrate-directed stereocontrol. Current development and refinement of catalytic asymmetric processes, however, show promise for the rise of the catalytic asymmetric Pauson-Khand reaction in the context of total synthesis. REFERENCES (1) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem. Soc., Perkin Trans. 1 1973, 977-981. (2) For preliminary work see (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem. Soc. D 1971, 36. (b) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem. Soc., Perkin Trans. 1 1973, 975-977. (3) (a) Buchwald, S. L.; Hicks, F. A. In Comprehensive Asymmetric Catalysis I-III; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-Verlag: Berlin, 1999; Vol. 2, pp 491-510. (b) Shibata, T. Adv. Synth. Catal. 2006, 348, 2328-2336. (4) Schore, N. E.; Croudace, M. C. J. Org. Chem. 1981, 46, 5436-5438. (5) Jeong, N.; Hwang, S. H.; Lee, Y. J. Am. Chem. Soc. 1994, 116, 3159-3160. (6) Hicks, F. A.; Kablaoui, N. M.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 9450-9451. (7) Kondo, T.; Suzuki, N.; Okada, T.; Mitsudo, T. J. Am. Chem. Soc. 1997, 119, 6187-6188. (8) Jeong, N. Organometallics 1998, 17, 3642-3644. (9) Hicks, F. A.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 11688-11689. (10) (a) Magnus, P.; Principe, L. M. Tetrahedron Lett. 1985, 26, 4851-4854. (b) Magnus, P.; Exon, C.; Albaugh- Robertson, P. Tetrahedron 1985, 41, 5861-5869. (11) a) Krafft, M. E.; Scott, I. L.; Romero, R. H.; Feibelmann, S.; Van Pelt, C. E. J. Am. Chem. Soc. 1993, 115, 7199-7207. (b) Gordon, C. M.; Kiszka, M.; Dunkin, I. R.; Kerr, W. J.; Scott, J. S.; Gebicki, J. J. Organomet. Chem. 1998, 554, 147-154. (12) a) Gimbert, Y.; Lesage, D.; Milet, A.; Fournier, F.; Greene, A. E.; Tabet, J. Org. Lett. 2003, 5, 4073-4075. (b) Banide, E. V.; Müller-Bunz, H.; Manning, A. R.; Evans, P.; McGlinchey, M. J. Angew. Chem. Int. Ed. 2007, 46, 2907-2910. (13) (a) Hiroi, K.; Watanabe, T.; Kawagishi, R.; Abe, I. Tetrahedron Lett. 2000, 41, 891-895. (b) Hiroi, K.; Wantanabe, T.; Kawagishi, R.; Abe, I. Tetrahedron: Asymmetry 2000, 11, 797-808. (14) Jeong, N.; Sung, B. K.; Choi, Y. K. J. Am. Chem. Soc. 2000, 122, 6771-6772. (15) Shibata, T.; Takagi, K. J. Am. Chem. Soc. 2000, 122, 9852-9853. (16) (a) Narasaka, K.; Shibata, T. Chem. Lett. 1994, 315-318. (b) Kent, J. L.; Wan, H.; Brummond, K. M. Tetrahedron Lett. 1995, 36, 2407-2410. (17) Wender, P. A.; Deschamps, N. M.; Gamber, G. G. Angew. Chem. Int. Ed. 2003, 42, 1853-1857. (18) Wender, P. A.; Croatt, M. P.; Deschamps, N. M. J. Am. Chem. Soc. 2004, 126, 5948-5949. (19) Wender, P. A.; Croatt, M. P.; Deschamps, N. M. Angew. Chem. Int. Ed. 2006, 45, 2459-2462. (20) Inagaki, F.; Mukai, C. Org. Lett. 2006, 8, 1217-1220. (21) Ishikawa, T.; Ishii, H.; Shimizu, K.; Nakao, H.; Urano, J.; Kudo, T.; Saito, S. J. Org. Chem. 2004, 69, 8133-8135. (22) Pallerla, M. K.; Fox, J. M. Org. Lett. 2007, 9, 5625-5628. (23) Hayashi, Y.; Miyakoshi, N.; Kitagaki, S.; Mukai, C. Org. Lett. 2008, 10, 2385-2388.